The present invention relates to a balanced amplifier circuit and a high-frequency communication apparatus, and an amplifier circuit (amplifier) in a high-frequency communication apparatus such as cell phones. More specifically, the present invention relates to a power amplifier.
It is widely known that methods for designing amplifier circuits are roughly divided into a single-ended type and a balanced type. The single-ended amplifier circuit, which is a two-terminal circuit having one input port and one output port, is a very generally and widely used circuit. The balanced amplifier circuit is structured such that two systems of single-ended amplifier circuits are disposed in parallel and driven in opposite phase before produced power is combined. The balanced amplifier circuit is used when high-efficiency amplification through push-pull operation is desired or suppression of even harmonics is desired.
The present invention focuses on the balanced amplifier circuit in high-frequency bands such as microwave bands and millimeter wave bands. As the prior art suitable for such high-frequency bands, a number of circuit modes are stated in JP 2001-267857 A for example. As it is difficult to cover and discuss all the circuit modes stated therein, two modes will be selected and described as typical examples.
FIG. 19 shows a circuit mode disclosed in the JP 2001-267857 A as an example selected as the first prior art, which combines and divides power with use of a distributed constant circuit-type 180-degrees Wilkinson power divider. In order to facilitate later comparison to the present invention, FIG. 19 in the present specification is illustrated so as to make up for deficient portions (e.g., bias circuit) in JP 2001-267857 A.
In FIG. 19, two systems of unit amplifiers 1911, 1912 are single-ended amplifier circuits respectively composed of input-side matching circuits 1907, 1909, semiconductor devices 1905, 1906, and output-side matching circuits 1908, 1910. In FIG. 19, FET (Field Effect Transistor) one-stage amplifier circuits are illustrated for easy understanding, although multi-stage amplifier circuits with use of other semiconductor devices (e.g., bipolar transistors) are also applicable. Drain terminals of the semiconductor devices 1905, 1906 are respectively connected to bias feed terminals 1903, 1904. Generally, as a low-pass filter circuit for preventing leakage of high-frequency signals, inductance components L1901, L1902 are respectively inserted in series immediately before the bias feed terminals 1903, 1904, and capacitance components C1901, C1902 are connected to between the bias feed terminals 1903, 1904 and grounds.
An electric signal given to an input port 1901 is divided in opposite phase and inputted into two systems of unit amplifiers 1911, 1912. The division of power in opposite phase is implemented by only one out of two outputs from a distributed Wilkinson power divider 1913 composed of a resistance R 1901 and two ¼-wavelength transmission lines T1903, T1904 being set to pass a 180-degrees phase-shift circuit composed of a half-wavelength transmission line T1901.
Power outputs in opposite phase from each other, which come from the two systems of unit amplifiers 1911, 1912 that are driven in opposite phase, are combined and then outputted from an output port 1902. The combination of power in opposite phase is implemented by only one out of two power outputs from two systems being set to pass a 180-degrees phase-shift circuit composed of a half-wavelength transmission line T1902 and then both the power outputs being combined in phase by a distributed Wilkinson power combiner 1914 composed of a resistance R 1902 and two ¼-wavelength transmission lines T1905, T1906.
FIG. 20 shows a circuit mode disclosed in the JP 2001-267857 A as an example selected as the second prior art, which combines and divides power with use of a lumped constant circuit-type 180-degrees Wilkinson power divider. In order to facilitate later comparison to the present invention, FIG. 20 in the present specification is illustrated so as to make up for deficient portions (e.g., bias circuit) in JP 2001-267857 A.
The circuit of FIG. 20 has a structure similar to that of FIG. 19 described before except that the distributed constant circuit is replaced with a lumped constant circuit, and shares the same principle of operation. In FIG. 20, two systems of unit amplifiers 2011, 2012 are single-ended amplifier circuits respectively composed of input-side matching circuits 2007, 2009, semiconductor devices 2005, 2006, and output-side matching circuits 2008, 2010. Drain terminals of the semiconductor devices 2005, 2006 are respectively connected to bias feed terminals 2003, 2004. As a low-pass filter circuit for preventing leakage of high-frequency signals, inductance components L2001, L2002 are respectively inserted in series immediately before the bias feed terminals 2003, 2004, and capacitance components C2001, C2002 are connected to between the bias feed terminals 2003, 2004 and grounds.
The distributed Wilkinson power divider 1913 in FIG. 19 is replaced with a lumped constant Wilkinson power divider composed of a resistance R2001, three capacitance components C2003, C2011, C2005, and two inductance components L2003, L2004 in FIG. 20. The distributed Wilkinson power divider 1914 in FIG. 19 is replaced with a lumped constant Wilkinson power divider composed of a resistance R2002, three capacitance components C2004, C2008, C2012, and two inductance components L2006, L2005 in FIG. 20. Two distributed phase-shift circuits T1901, T1902 in FIG. 19 are replaced with lumped constant phase-shift circuits 2013, 2014 in FIG. 20. The lumped constant phase-shift circuit 2013, which is a multi-stage circuit with capacitance components and inductance components disposed alternatively, is composed of two inductance components L2010, L2009 that are connected in series, and two capacitance components C2010, C2009 that are respectively connected to between input terminals of the inductance components L2010, L2009 and grounds. Moreover, the lumped constant phase-shift circuit 2014, which is a multi-stage circuit with capacitance components and inductance components disposed alternatively, is composed of two inductance components L2007, L2008 that are connected in series, and two capacitance components C2006, C2007 that are respectively connected to between output terminals of the inductance components L2007, L2008 and grounds.
Meanwhile, the present invention relates not only to a single amplifier circuit but also to the structure of a transmission system of a high-frequency communication apparatus incorporating the amplifier circuit. Eventually, description will be also given of the transmission system of a high-frequency communication apparatus in the prior art with reference to FIG. 17. The high-frequency communication apparatus in FIG. 17 is an example assuming a multimode and multiband cell phone such as the fourth generation cell phones. As the communication systems subject to multimode and multiband development, there are assumed four systems: 800 MHz-band cell phone; 1.9 GHz-band cell phone; 2.4 GHz-band wireless LAN; and 5.2 GHz-band wireless LAN. Inside a casing 1721, four systems of narrow-band transmission circuits are disposed in parallel corresponding to these four communication systems. A transmission signal in 800 MHz band is produced in a transmission signal source 1713, sent to a transmission amplifier 1705 through a transmission balun 1709 to be amplified, and emitted from an antenna 1701. A transmission signal in 1.9 GHz band is produced in a transmission signal source 1714, sent to a transmission amplifier 1706 through a transmission balun 1710 to be amplified, and emitted from an antenna 1702. A transmission signal in 2.4 GHz band is produced in a transmission signal source 1715, sent to a transmission amplifier 1707 through a transmission balun 1711 to be amplified, and emitted from an antenna 1703. A transmission signal in 5.2 GHz band is produced in a transmission signal source 1716, sent to a transmission amplifier 1708 through a transmission balun 1712 to be amplified, and emitted from an antenna 1704.
The reason why the communication baluns 1709 to 1712 are necessary is that as a trend of recent high-frequency circuit technologies, the transmission signal sources 1713 to 1716 are often realized with use of RFIC (Radio Frequency Integrated Circuit) technology. Generally, the inside of the RFIC is constituted from a balanced differential circuit, and therefore an output portion of the RFIC are generally composed of differential lines 1717 to 1720. On the contrary, most of the amplifier circuits in high-frequency bands (power amplifiers in particular) are a single-ended type, which necessitates the transmission baluns 1709 to 1712 for converting the differential lines (balance lines) to single-ended lines.
FIG. 18 is a view showing the operation of the circuit in FIG. 17 with schematic frequency spectrums. The 800 MHz band incorporates a narrow amplification band 1801 for the transmission amplifier 1705, the 1.9 GHz band incorporates a narrow amplification band 1802 for the transmission amplifier 1706, the 2.4 GHz band incorporates a narrow amplification band 1803 for the transmission amplifier 1707, and the 5.2 GHz band incorporates a narrow amplification band 1804 for the transmission amplifier 1708.
The balanced amplifier circuits (FIGS. 19 and 20) in the prior art had following two problems.
The first problem of the prior art is their large size. As pointed out in the JP 2001-267857 A, the prior art example of FIG. 20 achieved certain downsizing with heavy use of compacted lumped constant devices compared to the prior art example of FIG. 19. However, the prior art example of FIG. 20 falls short of sufficient downsizing because of following two reasons.
The first reason is the increased number of lumped constant devices. Regarding the problem of the number of the devices, discussion will be given later in this specification with a specific comparison to the present invention. The second reason is that lumped constant devices L2001, L2002, C2001, C2002 in the bias feed portion take extremely large device values for achieving a sufficient low-pass filter effect. For example, in the case of a MMIC (Monolithic Microwave Integrated Circuit) configuration, a large LC device value signifies large-area spiral inductance and MIM (Metal Insulator Metal) capacitance, resulting in considerable increase in a chip area.
The second problem in the prior art is that frequency bands in which a circuit performs a normal balance operation, i.e., the frequency bands in which two semiconductor devices 2005, 2006 are driven in opposite phase by almost exactly 180 degrees, become extremely narrow. This is a fundamental problem attributed to the circuit structure itself of FIG. 20 (and FIG. 19). The circuits of FIG. 20 and FIG. 19 depend on the 180-degrees phase-shift circuits T1901, T1902, 2013, 2014, and in such simple phase-shift circuits, 180-degrees phase-shift characteristics can be accurately implemented only in extremely narrow bands.
Whether or not the opposite-phase driving is accurately implemented is in most cases an issue relating to the nature of the balanced amplifier and therefore has importance. If the accurate opposite-phase driving is not required, then it is not necessary in first place to adopt the balanced amplifier structure, and so the normal single-ended amplifier structure is good enough. Typical cases where the balanced amplifier is required include, first, the case where it is desired to perform high-efficiency amplification through push-pull operation, and second, the case where it is desired to suppress second harmonic distortion and second harmonic spurious so as to achieve high-linearity amplification. In these cases, if the opposite-phase driving is not accurately implemented, the first case suffers a problem that efficiency as the amplifier is considerably deteriorated, while the second case suffers a problem that the second harmonic distortion and second harmonic spurious are not fully suppressed and end up to be leaked.
The aforementioned problem of the prior art that accurate balance operation is implemented only in a narrow band is extremely difficult to understand, and therefore in this specification, discussion will be given later with a specific comparison to the present invention.
In the high-frequency communication device (FIG. 17) in the prior art, following various problems have been caused by the fact that the baluns 1709 to 1712 are indispensable. First of all, a number of baluns 1709 to 1712 are required, which leads to increase in size and cost of the apparatus. Moreover, the insertion loss due to the baluns 1709 to 1712 is exerted on a transmission-system circuit which handles bulk power signals, which attributes to the increase in battery consumption. Further, as the most serious problem, difficulty for the baluns 1709 to 1712 to support wider bands attributes to inability to develop a multimode and multiband transmission system in the fullest sense.
FIG. 17 is a view assuming a multimode and multiband high-frequency wireless communication apparatus, which in actuality is composed of a plurality of transmission-system circuits arrayed simply inside a casing 1721, and therefore increase in size, weight and cost is unavoidable. In order to share and simplify the circuits, the circuits should be able to support wider bands so that a communication system in a certain band (e.g., 800 MHz) can receive a transmission signal in a different band (e.g., 5.2 GHz). However, many circuit components currently in use have difficulty to support wider bands as wide as 800 MHz to 5.2 GHz, and balun components (1709 to 1712) designed to be small in size and low in cost are included therein.